Method and apparatus for fabricating a composite structure consisting of a filamentary material in a metal matrix

ABSTRACT

A method and apparatus are provided for preparing a composite structure consisting of filamentary material within a metal matrix. The method is practiced by the steps of confining the metal for forming the matrix in a first chamber, heating the confined metal to a temperature adequate to effect melting thereof, introducing a stream of inert gas into the chamber for pressurizing the atmosphere in the chamber to a pressure greater than atmospheric pressure, confining the filamentary material in a second chamber, heating the confined filamentary material to a temperature less than the melting temperature of the metal, evacuating the second chamber to provide an atmosphere therein at a pressure less than atmospheric pressure, placing the second chamber in registry with the first chamber to provide for the forced flow of the molten metal into the second chamber to effect infiltration of the filamentary material with the molten metal, and thereafter cooling the metal infiltrated-filamentary material to form said composite structure.

Banker et al.

[ 1 Oct. 21, 1975 METHOD AND APPARATUS FOR FABRICATING A COMPOSITE STRUCTURE CONSISTING OF A FILAMENTARY MATERIAL IN A METAL MATRIX [75] Inventors: John G. Banker, Kingston; Robert C. Anderson, Clinton, both of Tenn.

[73] Assignee: The United States of America as represented by the United States Energy Research and Development Administration, Washington, D.C.

[22] Filed: July' 17, 1974 [21] Appl. No.: 489,215

[52] US. Cl. 164/62; 164/68; 164/105; 164/136; 164/254; 164/259; 164/332 [51] Int. Cl? B22D 19/02; B22D 23/00; B22D 27/16 [58] Field of Search 164/62, 65, 66, 68, 103, 164/105, 108, 109, 110, 136, 254, 259, 332, 335, 361

[56] References Cited UNITED STATES PATENTS 3,267,517 8/1966 Altermatt 164/62 X 3,690,367 9/1972 Daniels 164/338 X Primary ExaminerRobert D Baldwin Attorney, Agent, or FirmDean E. Carlson; David S. Zachry; Earl L. Larcher [57] ABSTRACT A method and apparatus are provided for preparing a composite structure consisting of filamentary material within a metal matrix. The method is practiced by the steps of confining the metal for forming the matrix in a first chamber, heating the confined metal to a temperature adequate to effect melting thereof, introducing a stream of inert gas into the chamber for pressurizing the atmosphere in the chamber to a pressure greater than atmospheric pressure, confining the filamentary material in a second chamber, heating the confined filamentary material to a temperature less than the melting temperature of the metal, evacuating the second chamber to provide an atmosphere therein at a pressure less than atmospheric pressure, placing the second chamber in registry with the first chamber to provide for the forced flow of the molten metal into the second chamber to effect infiltration of the filamentary material with the molten metal, and thereafter cooling the metal infiltrated-filamentary material to form said composite structure.

10 Claims, 7 Drawing Figures :1) SEE F I6. ID

PREHEAT U.S. Patent Oct. 21, 1975 Sheet 1 of3 3,913,657

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US. Patent Oct. 21, 1975 Sheet 3 of3 3,913,657

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METHOD AND APPARATUS FOR FABRICATING A COMPOSITE STRUCTURE CONSISTING OF A FILAMENTARY MATERIAL IN A METAL MATRIX This invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

The present invention relates generally to the fabrication of filament-metal matrix structures, and more particularly to a method and apparatus for infiltrating or impregnating a preformed filament body with molten metal to provide the filament body with a metal matrix.

Structures consisting of continuous or discontinuous filamentary material embedded in metal matrices possess several physical characteristics including high strength and high modulus-to-weight properties which can be readily tailored for meeting specific requirements where non-planar and multi-directional reinforcement properties are desired. For example, aircraft landing gear struts and aerospace nose cones could be readily constructed from the composites described herein. However, the utilization of these structures has been limited by shortcomings or drawbacks in the known fabrication apparatus and techniques. For example, manufacturing techniques which employ a hot pressing operation to eliminate voids in the structure and improve filament-matrix bonding after the components of the composite are in place cause large plastic strains to be introduced into the components. Inasmuch as these strains are necessary to move the metal matrix into the voids and improve the bonding between the filaments and matrix, such manufacturing processes are considered to be undesirable for fabricating threedimensionally reinforced structures in which relatively nonyielding filaments are arranged in nonparallel planes.

Efforts to provide for the fabrication of filamentmetal matrix structures wherein the voids are filled and components are bonded without excessively straining the in-place filaments include such techniques as casting. In casting operations such as previously employed, a preformed filament array is infiltrated with a molten matrix metal. The apparatus for infiltrating the filament array or structure have utilized the heat from the molten metal charge for preheating the filaments. In such cases, the filament body is submerged in the molten metal charge for the preheating operation prior to applying either a vacuum or pressure load for effecting the infiltration. Then after a temperature equalization is established between the filaments and the molten metal, the molten metal is drawn or forced by the vacuum or pressure load into the interfilament spaces or interstices between filaments. However, in these operations, a vacuum load of any significance cannot be drawn on the molten metal without causing excessive sublimation of the molten metal. Further, it was found that the filaments are extensively degraded when heated to or above the temperature required for melting the metal. Also, the contact between the filaments and the molten metal for the extended period of time necessary for the preheat and infiltration operations is a substantial contributor to the degradation of the filaments.

Accordingly, it is the primary aim or objective of the present invention to overcome or substantially minimize the above and other drawbacks or shortcomings encountered in the previously known techniques of fabricating filament-metal matrix structures. This goal is achieved by employing an apparatus and method wherein pressure and vacuum loadings are simultaneously applied toa molten metal charge and filament material body, respectively, to rapidly force the molten metal into the interstices of the filament material. In practicing this method, the filamentary body is heated to a temperature below the melting temperature required for melting the metal so as to substantially reduce filament degradation. Also, the simultaneous application of the vacuum and pressure loadings provides a structure wherein virtually all the interstices in the filament body are filled with the metal matrix.

Other and further objects of the invention will be obvious upon an understanding of the illustrative apparatus and method about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

Preferred embodiments of the invention have been chosen for the purpose of illustration and description of the subject apparatus and method. The preferred embodiments illustrated are not intended to be exhaustive or to limit the invention to the precise form or method steps disclosed. They are chosen and described in order to best explain the principles of the invention and their application in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications of the apparatus and method as are best adapted to the particular use contemplated.

Generally described, the pressure-vacuum, liquid metal infiltration apparatus of the present invention comprises a pair of contiguously disposed gasimpervious chambers, the interiors of which are separated from one another by an easily ruptured partition. One of the these chambers houses a filamentary body of the desired configuration and is coupled to a source of vacuum while the other chamber houses a charge of metal used, when molten, for infiltrating the filamentary body and is coupled to a source of pressurized inert gas. Both chambers are provided with separate heating mechanisms for melting the metal and heating of the filamentary body. When the filament body is preheated in the first chamber under a vacuum and the metal charge is melted in a pressurized atmosphere of inert gas in the other chamber, the partition separating the chambers is selectively ruptured to effect forced infiltration of the molten metal into" the filament body by simultaneously pulling the molten metal into the interstices of the filamentary body.

The configurations of the filament material-metal matrix structures are shown as cylindrical. However, it is to be understood that structures of many other configurations, such as solid or hollow cones, cubes, spherical, or any other desired geometrical configuration may be fabricated by practicing the present invention. Further, the filamentary material may be wound or loosely disposed in a preselected pattern, or, if desired,

.be randomly dispersed. Also, the filamentary material may be of any suitable material such as boron, boron nitride coated boron, silicon carbide, silicon carbide coated boron, tungsten, carbon, graphite, or metal coated graphite.

The metal used for forming the matrix may be of any suitable metal which can be infiltrated at a temperature below that which causes filament degradation or filament-matrix reaction. Examples of metals which may be suitable for use in the practicing of the present invention include aluminum, aluminum alloys, magnesium, magnesium alloys, and lead. The quantity of metal used for infiltrating the filamentary body is in excess of that required for completely filling the filamentary body. This excess may be easily removed from the surface of the impregnated filamentary body by employing conventional machining procedures.

The evacuation of the filamentary material prior to the metal impregnation is preferably at any desired subatmospheric pressure in the range of 200 to microns mercury, and is provided by coupling the apparatus to a suitable well-known vacuum source such as a vacuum pump. The pressure loading on the molten metal matrix material is preferably in the range of about 30 to 150 psig, and is provided by coupling the apparatus to a suitable pressurized source of an inert gas such as argon or helium.

In the accompanying drawing:

FIGS. 1 A through 1 F are schematic views showing one embodiment of the present invention useful for forming cylindrical filament material-metal matrix structures together with a flow diagram illustrating the method employed in the infiltration of the filament body with the metal matrix; and,

FIG. 2 is a schematic view showing another embodimant of the apparatus of the present invention.

Described more specifically and with reference to FIGS. 1A through 1F of the drawings, one embodiment of the filamentary body impregnating apparatus of the present invention is shown comprising an upright tubular housing 10 having a chamber 12 therein for receiving a charge of the metal 14 to be used as the metal matrix. The housing 10 is provided with a heating coil 16 disposed about the lower end thereof for heating the metal in chamber 12 to a molten state. As shown, the housing is divided into three vertically oriented zones: the lowermost zone 18 defines the chamber 12; the middle zone 20 contains the filament body as will be described below and is encompassed with heating elements 22 for heating the body of filamentary material prior to the metal infiltration step; and the uppermost zone 24 is a chill tower provided with suitable coolant conveying ducts 25 for effecting the rapid cool-down of the impregnated filamentary material. Movably disposed within the housing 10 is a yoke-shaped container or can 26 defining an annular chamber 28 which receives the body of filamentary material 30. This body of filamentary material is shown in FIG. 1A comprising metal filament strands 32 wound about a spool or mandrel 34 in the form of a cylinder. The loaded mandrel 34 is placed in the annular chamber 28 about a central body 36 affixed to the can 26 and having a cone-shaped top 38, as best shown in FIG. 1B.

With the filamentary body in place within the chamber 28 the lower end of the can is closed or sealed with a closure or membrane 40. This membrane 40 is formed of a material capable of withstanding the differential pressures employed in chambers 12 and 28 and yet is capable of being relatively easily ruptured to establish communication between these chambers at the desired time. A suitable membrane may be formed of stainless steel at a thickness of 5-10 mils. The upper end of can 26 is connected to a hollow conduit 42 which projects through the top of housing 10 and is coupled to a suitable source of vacuum, not shown. This conduit 42 is of a sufficient length so as to allow can 26 to traverse the full length of the interior of the housing 10 (FIG. 1C).

With the loaded can 26 disposed in zone 20 of housing 10, the chamber 28 in the can is evacuated and the heaters 22 energized to heat the filamentary body to a temperature of at least about C. less than the temperature of the molten metal depending upon the heat capacity of the matrix material. During this time the interior of the housing 10 is coupled to a source of inert gas, not shown, through conduit 44 for pressurizing the chamber 12 to a pressure in the aforementioned range. While pressurizing chamber 12 for a suitable time, the heaters 16 are energized to melt the metal charge 14. Preferably, this melting occurs immediately prior to the desired impregnation operation to assure that the filamentary body 30 will not be exposed to relativelyhigh temperatures for an inordinately long duration.

When the filamentary body 30 is heated to the desired temperature and the metal charge 14 is in a molten state, the can 26 is forcibly moved into chamber 12 by displacing the conduit 42 as generally shown in FIG. 1D. The can 26 penetrates the molten metal and contacts one or more pins or sharp projections such as shown at 46 at the bottom of chamber 12 so as to rupture the membrane 40 and instantly establish communication between chambers 12 and 28. Upon rupture of this membrane 40, the molten metal 14 is simultaneously pushed and pulled into the chamber 28 and into the interstices in the filamentary body 30. This pushing and pulling is effected by the differential pressure provided by subatmospheric pressure in the filamentary body and the pressurized atmosphere in chamber 12.

When the infiltration of filamentary body is completed, which nonnally takes about 1 to 5 seconds, the can 26 is withdrawn from zone 18 and moved into zone 24 (FIG. 1E) where the molten metal is rapidly chilled.

During the transfer of the impregnated filamentary body to zone 24, the surface tension between the filamentary material and the metal holds the matrix metal within the filamentary body.

After cooling the metal infiltrated filamentary body, the can 26 is removed from housing 10, and the mandrel 34 and any excess metal are removed from the impregnated filamentary body so as to provide the desired cylindrical filamentary material-metal matrix structure 48 as shown in FIG. 1F.

In a typical operation of the embodiment just described, a cylindrical filamentary body 30 of boron was formed by winding boron filaments on mandrel 34. The

cylindrical body had an outside diameter of 3.25

inches, a height of 3 inches, and a wall thickness of I 0.125 inch. The boron filament body 30 was enclosed in the annular can 26 and placed in the process housing 10 in zone 20. The chamber 28 within can 26 was placed in registry with a vacuum source and evacuated to a pressure of 200 microns mercury. As chamber 28 was evacuated and the filament body heated to 600C. in zone 20, the interior of housing 10 was pressurized with argon to a pressure of 35 psig. Also, during this time, a previously loaded charge 14 of magnesium metal was melted and heated to a temperature of 700C. in zone 18 of the housing 10. Upon completion of the required heating, evacuation and pressurization steps, the can 26 was lowered into the molten magnesium 14, with the thin-wall closure 40 of the can rupturing upon contact "with the pins 46 in the housing 10.

With the rupturing of the closure 40 the molten magnesium was forced into interstices of the filament body. The molten magnesium 14 .was uniformly infiltrated through the filament structure 30 with the pull from the low pressure inside the can 26 and the push from the argon pressure in the housing 10. The infiltrated structure was raised to the chill zone 24 where it was quenched and solidified.'Test data indicated the infiltrated filament structure contained 70 wt. percent filaments and had a tensile strength of 212,000 psi. Examination of cross sections taken from the metal filamentmetal matrix structure showed that the magnesium was infiltrated throughout the entire cross section of the structure and filled virtually every void in the filamentary body. From observation of fracture surfaces, the bonds between the filament and matrix appeared to be satisfactory. i

In FIG. 2, there is shown another embodiment of the apparatus of the present invention useful for forming cylindrical structures. In this embodiment two housings are employed. The lower housing 50 contains a chamber 52 which is encompassed by heating coils 54 and receives the cylindrical filament body 56 which may be fabricated as described above. The filament body 56 is disposed about a cylindrical structure 58 provided with a cone-shaped top 60 for facilitating the displacement of the molten metal into the filament body during the impregnation step. This chamber 52 is coupled to a suitable vacuum source through conduit 64 for effecting the evacuation of chamber 52 prior to the impregnation operation. The second housing 66 of this embodiment is connected to the first housing by a tube 67 and is provided with a central chamber 68 surrounded by heating coils 70 for melting a metal charge 72 contained within this chamber. The chambers 68 and 52 are separated from one another by rupturable membrane 74 which is broken by the movable pin 76 disposed in chamber 68 and projecting through the top of housing 66. The interior of the chamber 68 is also coupled to a source of pressurized inert gas through conduit 80 for pressurizing the interior of this chamber. The air in chamber 68 may be evacuated prior to backfilling with argon by coupling the chamber to a vacuum pump through conduit 82. The operation of this apparatus is similar to that described above for FIGS. 1A and 1B except that cooling of the impregnated filament body is provided by dropping the lower furnace from around the filament chamber and replacing it with a cooling chamber, such as a container of water for rapid cooling, for minimizing filament-matrix-contact at elevated temperatures.

It will be seen that the present invention provides a significant contribution to the art in that the degradation of the filament material being contacted with the molten metal has been successfully reduced. Further, the filamentary body is rapidly infiltrated with the molten metal so as to shorten the contact between the molten metal and the filament body at the higher temperatures of the molten metal. Also, evacuation of the filament body prior to the infiltration operation virtually, if not entirely, eliminates voids in the infiltrated structures.

As various changes may be made in the form, construction, and arrangement of the parts and method steps herein without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method for preparing a composite structure consisting of filamentary material within "a metal matrix, comprising the steps of confining the metal for forming the matrix in a first chamber, heating the confined metal to a temperature adequate to effect melting thereof, introducing a stream of inert gas into said chamber for pressurizing the atmosphere in said chamber to a pressure greater than atmospheric pressure, confining the filamentary material in a second chamber, heating the confined filamentary material to a temperature less than the temperature of said metal, evacuating said second chamber to a pressure less than atmospheric, placing said second chamber in registry with said first chamber to provide for the forced flow of the molten metal into said second chamber by the differential pressure in the chambers to effect infiltration of the filamentary material with the molten metal, and thereafter cooling the metal infiltrated-filamentary material to form said composite structure.

2. The method claimed in claim 1, wherein said first chamber is pressurized to a pressure in the range of about 30 to 150 psig, and wherein the pressure in said second chamber is in the range of about 10 to 200 microns mercury.

3. The method claimed in claim 2, wherein the step of placing said second chamber in registry with said first chamber is provided by rupturing a membrane providing a physical separation between the first and second chamber.

4. The method claimed in claim 2, wherein the temperature to which the filamentary material is heated in said second chamber is at least about C. less than the temperature of the molten metal in said first chamber.

5. The method claimed in claim 2, wherein the filamentary material is selected from the group consisting of boron, silicon carbide, silicon carbide coated boron, boron nitride coated boron, tungsten, carbon, graphite, and metal coated graphite.

6. The method claimed in claim 2, wherein the metal for forming the matrix is selected from the group of metals and alloys consisting of aluminum, aluminum alloys, magnesium, magnesium alloys, and lead.

7. An apparatus for fabricating a composite structure consisting of a body of filamentary material within a metal matrix, comprising a first chamber for housing a charge of metal used to provide said matrix, heating means encompassing said chamber for melting said metal, conduit means in registry with said chamber for conveying a stream of pressurized inert gas into said chamber, a second chamber disposed adjacent to said first chamber for housing said body of filamentary material, further heating means encompassing said second chamber for heating said body of filamentary material, further conduit means in registry with said second chamber for evacuating said chamber to a pressure less than atmospheric pressure, a membrane disposed between and physically separating the interior of said first chamber and said second chamber, and membrane rupturing means disposed in one of the chambers for rupturing said membrane to place the interior of said chambers in registry with one another.

8. The apparatus of claim 7, wherein both of said chambers are disposed in a single housing, said second about said zone for cooling the contents of said container when the latter is moved into said zone.

10. The apparatus claimed in claim 7, wherein said chambers are vertically oriented with said second chamber underlying said first chamber, and wherein said membrane-rupturing means comprises a movable member disposed in said second chamber. 

1. A method for preparing a composite structure consisting of filamentary material within a metal matrix, comprising the steps of confining the metal for forming the matrix in a first chamber, heating the confined metal to a temperature adequate to effect melting thereof, introducing a stream of inert gas into said chamber for pressurizing the atmosphere in said chamber to a pressure greater than atmospheric pressure, confining the filamentary material in a second chamber, heating the confined filamentary material to a temperature less than the temperature of said metal, evacuating said second chamber to a pressure less than atmospheric, placing said second chamber in registry with said first chamber to provide for the forced flow of the molten metal into said second chamber by the differential pressure in the chambeRs to effect infiltration of the filamentary material with the molten metal, and thereafter cooling the metal infiltrated-filamentary material to form said composite structure.
 2. The method claimed in claim 1, wherein said first chamber is pressurized to a pressure in the range of about 30 to 150 psig, and wherein the pressure in said second chamber is in the range of about 10 to 200 microns mercury.
 3. The method claimed in claim 2, wherein the step of placing said second chamber in registry with said first chamber is provided by rupturing a membrane providing a physical separation between the first and second chamber.
 4. The method claimed in claim 2, wherein the temperature to which the filamentary material is heated in said second chamber is at least about 100*C. less than the temperature of the molten metal in said first chamber.
 5. The method claimed in claim 2, wherein the filamentary material is selected from the group consisting of boron, silicon carbide, silicon carbide coated boron, boron nitride coated boron, tungsten, carbon, graphite, and metal coated graphite.
 6. The method claimed in claim 2, wherein the metal for forming the matrix is selected from the group of metals and alloys consisting of aluminum, aluminum alloys, magnesium, magnesium alloys, and lead.
 7. An apparatus for fabricating a composite structure consisting of a body of filamentary material within a metal matrix, comprising a first chamber for housing a charge of metal used to provide said matrix, heating means encompassing said chamber for melting said metal, conduit means in registry with said chamber for conveying a stream of pressurized inert gas into said chamber, a second chamber disposed adjacent to said first chamber for housing said body of filamentary material, further heating means encompassing said second chamber for heating said body of filamentary material, further conduit means in registry with said second chamber for evacuating said chamber to a pressure less than atmospheric pressure, a membrane disposed between and physically separating the interior of said first chamber and said second chamber, and membrane rupturing means disposed in one of the chambers for rupturing said membrane to place the interior of said chambers in registry with one another.
 8. The apparatus of claim 7, wherein both of said chambers are disposed in a single housing, said second chamber is defined by a container movable within and over essentially the full length of said housing, said membrane closes one end of said container, and wherein said membrane-rupturing means comprises a projection extending into said first chamber at the lowermost end thereof.
 9. The apparatus claimed in claim 8, wherein a cooling zone is within said housing at the upper end thereof, and wherein coolant-conveying means are disposed about said zone for cooling the contents of said container when the latter is moved into said zone.
 10. The apparatus claimed in claim 7, wherein said chambers are vertically oriented with said second chamber underlying said first chamber, and wherein said membrane-rupturing means comprises a movable member disposed in said second chamber. 